Nitrox course. What's the point?

[EFX]

I have no idea how to tell my Shearwater computers that I am breathing O2 at the surface. Please tell me how to do that.

You can't select O2 for the surface. I have a Perdix also.

 

[Belzelbub]

Okay, I assumed all deco schedules reduced a critical amount of nitrogen, but you’re say because the gas is enriched it leaves less residual nitrogen and so lessens the surface interval, if that’s true would breathing 100% O2 on the surface also reduces the surface interval?

Yes. This is correct. Keep in mind that for a relatively mild EAN mix, the difference in residual N2 off-gassed during ascent is pretty small. However, during a surface interval, the difference is more significant.

In fact, I may be oversimplifying, but this is largely the issue with some early Uwatec Aladdin Air X dive computers that resulted in a lawsuit. The computers in question were manufactured around 1995, so the problem was fixed a long time ago. Anyway, the problem with the algorithm in those computers in Nitrox mode was that they assumed the surface interval was spent breathing the same mix as was used during the dive. On subsequent dives, this resulted in calculating NDL that was longer than it should have been.

 

[Degenerate]

Ok, "millions" is of course just a rhetoric figure.
But CMAS or BSAC affiliated clubs make diving courses in almost every european city, all the year around, at prices which are roughy half of PADI, and borrowing you the equipment for free for the course. People interested into diving usually choose the cheapest option.
PADI is strong only in diving resorts, when people are on holidays and find that in their hotel or resort there is a diving center, and they want to try it. When back at home, usually people understand how poor was the training, and if they did like the experience, they continue with a longer, serious course in the local diving club. So yes, PADI makes a good business on holidays location, not in European towns.
I have no figures comparing the number of OW certifications released every year in Europe, but I suppose that the two numbers are not very different.
In UK I am told that BSAC is well ahead of PADI in the number of divers trained each year...

I don't know how it is in Europe as a whole, but here in Sweden the amount of people who get their certs through clubs connected to CMAS (or other similar) are a drop in the ocean compared to the PADI/SSI ones.
Our club is one of the largest in the country and typically they only get 2 classes done per year, with 4-6 students per class.
To put that in perspective, the local dive shop certifies 4-8 students every other week for 2/3 of the year.
IMO, people interested in diving do not usually choose the cheapest option, they'll go for the option that will get it done the fastest, because most people don't have the patience to spend 1-2 months on a class because the instructor is volunteering his time and can't spend every weekend teaching for free.

 

[boulderjohn]

I wrote about my wish that the computer could be set for the gas you are breathing ocne you are on the surface:

When I reach the surface, my Shearwater computer assumes I am breathing air, and it gives me no other option. It would be a piece of cake for them to add code that allows me to identify a different breathing mix and track my tissue off-gassing accordingly, just as it does under water, but they will not do that, I assume because it has not been studied.

You responded that being on the surface does not matter--you are "'still diving' but on the surface."

The computer doesn't care you are on the surface. (The Shearwater Perdix stores the pre-dive surface pressure). For all practical purposes when you are back on the surface the computer continues to calculate tissue loading. You are essentially "still diving" but on the surface. T

I then asked how to tell my computer that I am breathing something other than air when I am on the surface, and you wrote:

You can't select O2 for the surface. I have a Perdix also.

OK, so if the computer doesn't care I am on the surface, how is it possible that it assumes I am breathing air and will not let me tell it anything different? You said it was still diving--I can change gases while I am diving.

 

[EFX]

The computer does not calculate NDL using any assumed amount of offgassing on ascent. How could it? What value of ascent do we use? 20 fpm? 30 fpm? 60 fpm? You are correct in that two divers, one using air and one using EAN36, if dove to the same NDL would have an equal N2 loading. On ascent to the surface the nitrox diver would have less residual N2 in their tissues because of "wash out" due to the higher O2. So, these are two different issues. NDL is defined as the time remaining at the current depth before mandatory deco stops are needed. Or, to put it another way it is the time remaining where a diver may ascend directly to the surface. There is no ascent implied in the definition. In my spreadsheet the algorithm I use does not use ascents. I posted the algorithm in another thread.

I want to clarify the issue between NDL calculations and ascent rates. In my Excel spreadsheet I use the Schreiner equation to calculate tissue loading for 16 tissue compartments (TC) for each particular depth the user enters. (Another form of the equation which produces the exact results is called the Buhlmann equation.) The algorithm I use for calculating NDL uses the inert gas loading in the TC's to calculate a ceiling. The ceiling is the distance from the surface. For NDL dives the ceiling is negative. For deco dives the ceiling is positive (denoting a stop depth below the surface) and the ceiling for the surface is zero. The algorithm starts at 0 time and keeps adding one minute until the ceiling becomes 0. The total number of minutes accumulated when the ceiling reaches 0 is the NDL time remaining at that depth. For each iteration of increasing time a new TC pressure is calculated and then a new ceiling based on that TC pressure. The TC with the lowest ceiling (less negative for NDL or more positive for deco dives) becomes the controlling compartment.

Notice that in the above description there is no mention of ascent rate, time, or offgassing. The ascent offgassing is taken into account when an NDL is calculated for a newer shallower depth. At the new depth, newly calculated TC pressures are used to determine new ceilings which in turn lead to newly calculated NDL time. So, in effect ascent offgassing is taken into consideration for calculating NDL but the NDL calculation itself involves no ascent rates. I hope this explanation is clearer.

 

[EFX]

OK, so if the computer doesn't care I am on the surface, how is it possible that it assumes I am breathing air and will not let me tell it anything different? You said it was still diving--I can change gases while I am diving.

It assumes you are breathing air because that is what most if not all divers do when they reach the surface; they take their regulators out and breath air. From the standpoint of the calculations the computer doesn't care that you are underwater, on the surface, or 10,000 ft above sea level. But, it does "know" that the in-water dive has ended and you are on the surface, and it restricts your gas to air. (The Shearwater Perdix manual has some information on this.) I posted my guess why Shearwater doesn't allow another gas on the surface. You should ask them why?

 

[Storker]

None of the WRSTC members teach decompression as a part of OW training.

I don't think anyone here has claimed that. PADI OW is roughly the same as CMAS 1* or BSAC OD. AFAIK neither of those classes teach staged deco. But unlike PADI, as you climb the ladder, you get to learn deco theory and practice and are certified for simple backgas deco without "going tech" with all those bells, whistles and woo. Both CMAS' and BSAC's equivalent to PADI DM (3* or DL, respectively) qualify you for simple deco dives.

 

[MichaelMc]

It assumes you are breathing air because that is what most if not all divers do when they reach the surface; they take their regulators out and breath air. From the standpoint of the calculations the computer doesn't care that you are underwater, on the surface, or 10,000 ft above sea level. But, it does "know" that the in-water dive has ended and you are on the surface, and it restricts your gas to air. (The Shearwater Perdix manual has some information on this.) I posted my guess why Shearwater doesn't allow another gas on the surface. You should ask them why?

One way to add a surface O2 feature would be with a time out. You could designate O2 for say 15 minutes but would have to reaffirm O2 use every 15 minutes when a timer went off. Avoids the left on O2 overnight issue without being too difficult for intentional surface O2 off-gassing. Available in some tech mode only.

 

[EFX]

Yes a timer would help. Or, set the computer for 100% O2, immerse it in 3 ft of water, then breath your O2. The computer will start a new dive but at least it will calculate new tissue loadings while you are on surface O2. The added pressure from the water shouldn't make any difference. I don't know how you could do that traveling in a car though. You could use a small pressurized container to house the computer small enough to take in the car.

 

[Belzelbub]

While there is a benefit to breathing pure O2 at the surface, I can’t see a good reason for a DC manufacturer to add that as an option. Most divers just breathe atmospheric air at the surface. There really is no downside to breathing a higher O2 mix at the surface than the computer thinks. Yes, the NDL on the next dive will show lower, but that just increases safety margin. There is, however, a significant downside to breathing air when the computer thinks you are breathing a higher %.

If the extra time really makes a difference, the diver is being very aggressive, and probably heading for DCS anyway.